Slip casting is a process in which a low viscosity, high solids slip is dewatered in a plaster mold.
The properties of the casting slip, and the properties of the bodies formed from it, will vary with such factors as the particle size distribution of the materials in the slip, the chemical nature(s) of the materials in the slip, the surface area of such materials, and the like. Even a minor change in the physical or chemical properties of one of the components of the slip may substantially affect the properties of the slip.
Many of the prior art casting slips have been made by blending ball clay, kaolin clay, non-plastics, and water. Thus, for example, a porcelain casting slip frequently was prepared by blending from about 25 to about 39 parts of ball clay, from about 11 to about 25 parts of kaolin clay and from about 40 to about 55 parts of non-plastic material.
In general, relatively unsophisticated equipment and workers were used to prepared the casting slips. The materials often were charged into a tank equipped with a mixer. Often one worker would insert a stick into the tank while another worker would fill the tank to a specified stick height with one or more of the reagents. Needless to say, this practice did not produce casting slips whose properties were substantially constant from batch to batch.
In order to make the process of preparing the casting slip easier to use, applicant invented a ball clay slurry. This slurry, which is described in applicant's U.S. Pat. Nos. 4,812,487, 4,812,428, and 4,880,759, provided the manufacturer of the casting slip with a material whose properties were substantially identical from batch to batch.
Although the ball clay slurry described in applicant's patents substantially simplified the casting slip process, it still was necessary to mix this slurry with kaolin clay (in either slurry and/or powder form) and with non-plastic material. Although the opportunity for operator error was substantially reduced with the use of applicant's slurry, many errors could still be (and were) made by the operators.
In his prior patents, applicant broadly described a mixture of a ball clay slurry and a kaolin clay slurry; see, for example, columns 33-35 of U.S. Pat. No. 4,880,759. Thus, in said U.S. patent, it is disclosed that the weight/weight ratio of the ball clay and the kaolin clay in the slurry may range from 0.11 to 9.
Many thousands of mixtures of ball clay and kaolin clay are described in U.S. Pat. No. 4,880,759. Many of these mixtures, however, do not possess the required combination of properties needed to prepare a commercially acceptable casting slip.
As is disclosed in U.S. Pat. No. 4,880,759, slip casting is the process in which a low viscosity, high solids content slip is dewatered in a plaster mold. This dewatering must be accompanied by a simultaneous gellation of the colloidal fraction of the particles within the body. These two processes, dewatering and gellation, re both necessary and, in some slips, are frequently opposed to each other. They perform separate functions and, only when balanced in exactly the correct relationship, do they together provide optimum casting behavior.
It is an object of this invention to provide a clay slurry whose unique combination of properties make it suitable for mixing with non-plastic material to produce a casting slip with a commercially acceptable properties.
It is another object of this invention to provide a clay slurry which, when mixed with non-plastic material, forms a slip which may be used to prepare bodies with improved strength properties.
It is another object of this invention to provide a process for making a novel clay slurry.